For a housing or decoration of an electronic device such as a portable phone, a material is appropriately selected from materials such as resin and metal and is used in consideration of various factors such as decorativeness, scratch resistance, workability, and cost.
In recent years, there have been attempts to use, as a material for housing, a glass that has not been used hitherto. According to Patent Reference 1 (JP-A 2009-61730 (KOKAI)), by forming the housing itself from a glass in an electronic device such as a portable phone, it is possible to exhibit a unique decorative effect with transparency.
The housing or decoration of an electronic device for portable use such as a portable phone is required to have high strength in consideration of breakage by an impact of dropping when in use or contact scratches due to long-term use.
As a method to increase strength of the glass, a method of forming a compressive stress layer on a glass surface is generally known. Representative methods to form the compressive stress layer on a glass surface are an air-cooling tempering method (physical tempering method) and a chemical strengthening method. The air-cooling tempering method (physical tempering method) is performed by rapidly cooling such as air cooling a glass plate surface heated to a temperature near a softening point. On the other hand, the chemical strengthening method is to replace alkali metal ions (typically, Li ions, Na ions) having a smaller ion radius existing on the glass plate surface with alkali ions (typically, Na ions or K ions for Li ions, or K ions for Na ions) having a larger ion radius by ion exchange at temperatures lower than or equal to a glass transition point.
For example, in general, the glass for decoration as described above is often used with a thickness of 2 mm or less. When the air-cooling tempering method is employed for such a thin glass plate, it is difficult to assure a temperature difference between the surface and the inside, and hence it is difficult to form the compressive stress layer. Thus, in the glass after being tempered, the intended high strength characteristic cannot be obtained. Further, in the air-cooling tempering, due to variation in cooling temperature, there is a great concern that the flatness of the glass plate is impaired. The concern that the flatness is impaired is large in a thin glass plate in particular, and there is a possibility of impairing texture aimed by the present invention. From these points, it is preferred that the glass plate be tempered by the latter chemical strengthening method.
Further, in the housing or decoration of an electronic device such as a portable phone, a glass having a dark color tone such as black or gray is widely used which does not strongly emphasize the presence of the device itself, and by which firmness and luxuriousness can be obtained simultaneously. Among others, a gray-based color tone gives a soft impression and makes a stain due to an extraneous matter on the surface less noticeable, and thus is widely applied to a housing or the like of an electronic device.
It has been known that a glass can be chemical strengthened and exhibits a dark color by Patent Reference 2 (JP-B 1970-16112 (KOKOKU)). The glass described in Patent Reference 2 is an aluminosilicate glass containing a high concentration of iron oxide.